1. Field of the Invention
The present invention relates to a depleted UF6 processing plant for processing depleted UF6 by converting UF6 into U3O8, and a method for processing depleted UF6.
2. Discussion of the Background
The proportion of depleted UF6 accumulated in an uranium enrichment plant amount to nearly 90% of the UF6 starting material, and it is mostly stored by filling in a UF6 cylinder that is a cylindrical sealed storage vessel. However, since this substance is almost permanently stored, there arises a management problem of maintaining the vessel with a large quantity of depleted UF6 from corrosion over an extended period of time, as well as waste of resources and economical deficiencies caused by a vast amount of fluorine resources being stored in the form of UF6.
A large amount of depleted UF6 containing a low concentration of U235 is accumulated in the enrichment process of U235 in the uranium enrichment plant when U235 is enriched using UF6 produced from natural uranium or recovered UF6 as a starting material. To solve the problems described, the inventors of the present invention proposed a method for processing depleted UF6 by converting depleted UF6 containing a low concentration of U235 into U3O8 by a dry vapor-phase reaction method (Japanese Unexamined Patent Publication No. 11-79749). In the method for processing depleted UF6, anhydrous hydrogen fluoride is extracted as a by-product with concentrated sulfuric acid and anhydrous hydrogen fluoride is separated from dilute sulfuric acid by distillation; dilute sulfuric acid is further distilled and concentrated so as to separate dilute hydrofluoric acid from concentrated sulfuric acid; this concentrated sulfuric acid is recycled to the extraction and concentration step while dilute hydrofluoric acid is further distilled so as to separate it into azeotropic hydrofluoric acid and water that contains a small amount of hydrofluoric acid; and azeotropic hydrofluoric acid is mixed with dilute hydrofluoric acid in the distillation and concentration step to improve recovery of hydrogen fluoride for recycling in the nuclear facilities.
However, two distillation columns and one concentration column are required to regenerate hydrogen fluoride during the processing of depleted UF6 described above in the nuclear facilities. To recycle hydrogen fluoride in the existing nuclear facilities, additional equipments for the recycling should be installed, resulting in not matching supply and demand of anhydrous hydrogen fluoride. Accordingly, it is also desirable to recover and store hydrogen fluoride generated as a by-product when depleted UF6 is converted into U3O8 since it can be readily recycled.
The method for recovering and storing fluorine known in the art includes processes of forming calcium fluoride by a fixing reaction of fluorine to calcium, followed by storage of calcium fluoride. The inventors of the present invention proposed a method for recovering granular calcium fluoride by allowing a solution mainly containing hydrogen fluoride to contact granular calcium carbonate, and an equipment to be used for the method (Japanese Unexamined Patent Publication No. 10-330113). This equipment includes a storage tank for storing a solution containing 10 to 60% of hydrogen fluoride, a first cooler for cooling the solution stored in the storage tank to 0 to 5° C., a reaction tank for forming a solution containing granular calcium fluoride by adding granular calcium carbonate to the solution at a temperature of 0 to 5° C. fed from the storage tank, and a solid/liquid separator for separating granular calcium fluoride from the solution containing it. This method is so devised that fluorine is recovered with a high yield by forming calcium fluoride by cooling the reaction solution to 0 to 5° C. in the first cooler.
However, in the equipment in Japanese Unexamined Patent Publication No. 10-330113, the hydrogen fluoride gas generated as a by-product of the conversion of UF6 to U3O8 is turned into an aqueous hydrogen fluoride solution containing 10 to 60% of hydrogen fluoride once, in order to recover hydrogen fluoride as a by-product. The foregoing conversion process requires installation of additional facilities. Also, in the conventional process described above, the recovery work becomes much complicated if the by-product hydrogen fluoride generated in the conversion of UF6 into U3O8 could not be recovered. Also, there is a drawback that calcium fluoride formed in the recovery of hydrogen fluoride tends to be a fine powder.